The Wildland Urban Interface

About the WUI

Population deconcentration in the U.S. has resulted in rapid development in the outlying fringe of metropolitan areas and in rural areas with attractive recreational and aesthetic amenities, especially forests. This demographic change is increasing the size of the wildland-urban interface (WUI), defined as the area where structures and other human development meet or intermingle with undeveloped wildland. The expansion of the WUI in recent decades has significant implications for wildfire management and impact. The WUI creates an environment in which fire can move readily between structural and vegetation fuels. Its expansion has increased the likelihood that wildfires will threaten structures and people.

WUI Maps and GIS data

WUI maps are intended to illustrate where the WUI was located in 1990, 2000, and 2010. We map two types of WUI: intermix and interface. Intermix WUI are areas where housing and vegetation intermingle; interface WUI are areas with housing in the vicinity of contiguous wildland vegetation.

WUI GIS data were designed to provide a spatially detailed national assessment of the Wildland Urban Interface (WUI) across the coterminous U.S. to support inquiries into the effects of housing growth on the environment, and to inform both national policy and local land management concerning the WUI and associated issues. These data are useful within a GIS for mapping and analysis at national, state, and local levels and are available for download by state as compressed ESRI Shapefiles (.shp), or for the entire U.S. as a compressed file geodatabase (.gdb).

The definition we used to map the WUI originated in the Federal Register (66:751, 2001) report on WUI communities at risk from fire (USDA & USDI, 2001), and Tie and Weatherford’s 2000 report to the Western Governor’s Association on WUI fire risk.

Housing density: Housing density information was derived from U.S. Census data. Analysis was conducted at the finest demographic spatial scale possible, Census blocks, from the 1990, 2000, and 2010 Census. All measures of housing density are reported as the number of housing units per square kilometer.

Landcover: We utilized the National Land Cover Dataset (NLCD 2006 for WUI 2010, and NLCD 1992-2001 Change Product for WUI version 3), a satellite data classification produced by the USGS with 30m resolution and available for the entire U.S. (Vogelmann et al. 2001) to identify 'wildlands'. Our definition of 'wildlands' encompasses a range of management intensities. NLCD classes that we included as 'wildlands' are forests (coniferous, deciduous and mixed), native grasslands, shrubs, wetlands, and transitional lands (mostly clear-cuts). We exclude orchards, arable lands (e.g., row crops) and pasture.

The Wildland-Urban Interface (WUI): WUI is composed of both interface and intermix communities. In both interface and intermix communities, housing must meet or exceed a minimum density of one structure per 40 acres (16 ha). Intermix communities are places where housing and vegetation intermingle. In intermix, wildland vegetation is continuous, more than 50 percent vegetation, in areas with more than 1 house per 16 ha. Interface communities are areas with housing in the vicinity of contiguous vegetation. Interface areas have more than 1 house per 40 acres, have less than 50 percent vegetation, and are within 1.5 mi of an area (made up of one or more contiguous Census blocks) over 1,325 acres (500 ha) that is more than 75 percent vegetated. The minimum size limit ensures that areas surrounding small urban parks are not classified as interface WUI.

Buffer Distance for Interface: The California Fire Alliance (2001) defined "vicinity" as all areas within 1.5 mi (2.4 km) of wildland vegetation, roughly the distance that firebrands can be carried from a wildland fire to the roof of a house. It captures the idea that even those homes not sited within the forest are at risk of being burned in a wildland fire. We adopt this buffer distance to identify interface areas. With minimum housing densities, vegetation types, and interface buffer distances determined, the operational definition of the WUI is complete.

Acknowledgements

We gratefully acknowledge financial support for this research by the Northern Research Station, the Pacific Northwest Forest Inventory and Analysis Program, and the Northern Global Change Program of the USDA Forest Service under the National Fire Plan.